(a) Field of the Invention
This invention relates to improved color-developing compositions for pressure-sensitive recording sheet material of the single-sheet type.
(b) Description of the Prior Art
Pressure-sensitive recording sheet materials are generally divided into two groups: the double-sheet and the single-sheet type. Pressure-sensitive recording sheet materials of the double-sheet type comprise a first sheet (generally known as "coated back sheet") coated with microcapsules containing an electron donative organic compound (commonly called "pressure-sensitive dye") dissolved in a non-volatile and hydrophobic organic solvent and a second sheet (generally known as "coated front sheet") coated with an aqueous coating composition containing an electron attractive color developer, these sheets being disposed with the coated surfaces thereof facing each other. On the other hand, pressure-sensitive recording sheet materials of the single-sheet type (commonly called "self-contained papers") comprise a single sheet having one surface coated both with a layer of microcapsules and with a layer of color developer.
In the above-described pressure-sensitive recording sheet materials are used one or more pressure-sensitive dyes selected from (1) triphenylmethane dyes such as Crystal Violet Lactone, (2) methylene blue dyes such as Benzoyl Leucomethylene Blue, (3) fluoran dyes such as Rhodamine Lactam, (4) leucoauramine dyes, (5) spiropyran dyes, and the like.
On the other hand, the electron attractive color developers used therein include (1) inorganic solid acids such as acid clay and activated clay, (2) certain phenol-formaldehyde polymers, (3) metallic salts of substituted salicylic acids, and the like. Among these color developers, certain phenol-formaldehyde polymers and particularly p-phenylphenol-formaldehyde polymers are widely used in pressure-sensitive recording sheet materials of the double-sheet type because of (1) their excellent color-developing performance, (2) the good application properties (or rheological properties) of the aqueous coating color prepared therefrom and (3) the good water resistance of the sheets coated therewith.
In the prior art, however, it was quite impractical to use such p-phenylphenol-formaldehyde polymers as the color developer for pressure-sensitive recording sheet material of the single-sheet type because they tend to cause a remarkable degree of backgrounding (or spontaneous color development) during manufacture and/or handling.
The present inventors have previously proposed the use of certain p-alkylphenol-phenol-formaldehyde copolymers as the color developer for pressure-sensitive recording sheet material of the single-sheet type. They are copolymers obtained by reacting one or more p-alkylphenols having the general formula ##STR1## where R is an alkyl radical containing from 1 to 12 carbon atoms, and phenol with formaldehyde in the presence of an acid catalyst, and are now finding wide acceptance.
When used as the color developer for pressure-sensitive sheet material of the single-sheet type, the above-described p-alkylphenol-phenol-formaldehyde copolymers show great advantages such as (1) excellent color-developing performance, (2) good resistance to staining by moist heat (or a low degree of spontaneous color development), and (3) good resistance to yellowing by light. However, these copolymers and self-contained papers fabricated therewith are still liable to yellowing by air oxidation. Thus, there is an eager desire for an improvement which can solve this problem.